The present invention relates to a system and method for monitoring the rate of etching of a semiconductor wafer. In particular, the present invention relates to a system and method for monitoring the rate of etching of a semiconductor wafer using interferometry.
In the manufacture of semiconductor devices, for example, integrated circuits or flat panel displays, layers of materials are alternately deposited onto, and etched from, a substrate surface. As is well known in the art, the etching of the deposited layers may be accomplished by wet etching, dry etching and other techniques. The dry etching technique includes, for example, plasma chemical etching and plasma reactive ion etching.
In the dry etching technique, the etching of the semiconductor typically takes place inside a plasma processing chamber. The semiconductor""s outer layer is coated with an appropriate photoresist mask or hard mask. Plasma gas etches the areas of the outer layer not protected by the mask to form a desired pattern in the outer layer.
To avoid etching too deeply during this process, the rate of etching must be monitored. Interferometry frequently is used for this purpose. In general, when using interferometry, the substrate""s outer layer is illuminated using a light source. This light source may be, for example, a laser, a tungsten/halogen lamp or a plasma emission. A photodetector collects light reflected from this outer layer and the trenches being etched within this layer. In a typical plasma processing chamber, both the light source and the photodetector are disposed outside of the chamber to avoid exposing these devices to the corrosive etching environment within the chamber. The plasma processing chamber, therefore, employs a transparent observation window through which the illuminating beams and the reflected beams pass.
The photodetector monitors the reflected light for the repetitive maximum intensity and minimum intensity resulting from, respectively, constructive interference and destructive interference of the reflected light during the etching process. As shown in FIG. 1, the light reflected from the outer layer and the trenches exhibits a maximum intensity or a minimum intensity each time the depth of the trenches changes by an amount xcex94d equal to one half the wavelength xcex of the incident light. Although not shown in FIG. 1, this repetitive pattern generally attenuates over time with the trenches"" increasing depth. The period T of each maximum cycle and minimum cycle of the reflected light, therefore, represents a change in the depth of the trenches proportional to xcex/2. The rate of etching R can be represented, therefore, as follows:
R=xcex94d/T=xcex/2T
Using an appropriate logic circuit, the photodetector determines the depth to which the outer surface has been etched based upon the wavelength of the incident light and the time period between maximum intensities or minimum intensities of the reflected light.
In order for the intensity of the light reflected from the bases of the trenches to be adequate to create a detectable interference pattern with the light reflected from the substrate""s outer layer, it is well established in the art to select the wavelength of the incident light such that the trenches act as waveguides for transmission of the light incident upon them. As is also well known in the art, a waveguide transmits light or other electromagnetic radiation only if the wavelength of the radiation is below a certain cutoff value (the frequency of the radiation is above a certain cutoff value). The high attenuation losses within the waveguide for electromagnetic radiation having wavelengths above the cutoff value (frequencies below the cutoff value) prevent a standing wave from existing within the waveguide.
The cutoff wavelength xcexc for the radiation depends upon the geometry of the waveguide. For a rectangular waveguide, radiation can be transmitted along the waveguide only if the radiation""s wavelength is less than twice the long edge b of the waveguide as shown in FIG. 2 (xcexc=2b). For a circular waveguide, the cut off wavelength xcexc is 1.71 times the diameter of the waveguide (xcexc=1.71d).
Manufacturers of semiconductors are continuously striving to fabricate more and more transistors on a single semiconductor wafer. As a result, the size of the trenches etched in the wafers is becoming progressively smaller. The width b of rectangular trenches or the diameter of circular trenches (critical dimension) in many cases is now less than 100 nanometers(nm). Trenches having such a small size substantially complicate the use of interferometry for measuring the rate of etching.
For circular trenches having a diameter of 100 nanometers or less, the wavelength of the incident light must be less than 171 nanometers for the trenches to act as waveguides. The frequency of such electromagnetic radiation is in or about the ultraviolet range. At such small wavelengths (high frequencies), the radiation is quickly absorbed by both the atmosphere and the substrate, notwithstanding that the trenches act as waveguides. As a result, the interferometric equipment used at such small wavelengths must be specially designed including being housed within a vacuum or within an atmosphere purged with nitrogen to overcome absorption of the radiation by the atmosphere. The use and manufacture of such equipment is extremely complicated and expensive. Even using such equipment, the substantial absorption of the electromagnetic radiation by the substrate often results in a poor interferometric signal during etching.
We have discovered that the use of interferometry to measure the etching rate of small trenches on semiconductor wafers and other substrates is substantially enhanced by using light or other electromagnetic radiation having wavelengths substantially longer than the cutoff wavelength for the trenches, i.e., substantially longer than the wavelengths at which the trenches act as waveguides for the radiation. Although the use of wavelengths above the cutoff wavelength prevent the trenches from acting as waveguides to support transmission of the electromagnetic radiation to the bases of the trenches, the substrate at such wavelengths is far more transparent to the radiation. As a result, for small trenches, substantially more radiation passes into the substrate and actually reaches the bases of the trenches than at wavelengths below the cutoff wavelength. Also, the substrate remains sufficiently reflective at the longer wavelengths for substantial radiation still to be reflected from both the substrate""s outer layer and the bases of the trenches. As result, notwithstanding that the trenches do not act as waveguides for transmitting the incident radiation, substantial constructive and destructive interference nevertheless occurs to produce a strong interferometeric signal for monitoring the etching rate.
A method of measuring the rate of etching of trenches on a substrate in accordance with the present invention includes transmitting onto the substrate incident electromagnetic radiation having a wavelength above the wavelength at which the trenches act as waveguides for the radiation; collecting reflected electromagnetic radiation from the substrate; detecting a repetitive pattern of maximum intensities and minimum intensities of the reflected electromagnetic radiation during the etching; and determining the rate of etching based upon the wavelength of the incident electromagnetic radiation and the time period of the pattern.
For a circular or rectangular trench, the wavelength of the incident electromagnetic radiation preferably is, respectively, greater than 1.71 times the diameter of the trench and 2.00 times the length of the long edge of the trench. The critical dimension of the trenches preferably is 200 nm or less, and, for such trenches, the wavelength of the incident electromagnetic radiation preferably is 470 nm or greater.
The substrate preferably is a semiconductor wafer selected from the group consisting of silicon (Si), germanium (Ge) and the compound semiconductors including, but not limited to, gallium arsenide (GaAs), gallium nitride (GaN), indium phosphide (InP) and gallium indium phosphide (GaInP). The substrate preferably is housed within an etching chamber, and the incident electromagnetic radiation and the reflected electromagnetic radiation preferably are transmitted through a window in the etching chamber. The incident electromagnetic radiation preferably is transmitted from a source of radiation selected from the group consisting of a diode laser, a tungsten/halogen lamp and a helium/neon light.
The transmitting of the incident electromagnetic radiation preferably comprises transmitting this radiation through a focusing lens onto the substrate, and the collecting of the reflected radiation preferably comprises collecting this radiation from a collecting lens.
The detecting of the repetitive pattern of maximum intensities and minimum intensities preferably comprises detecting this pattern using a photo detector. The rate of etching preferably is proportional to xcex/2T, where xcex is the wavelength of the incident light and T is the time period between consecutive maximum intensities of the reflected light or consecutive minimum intensities of the reflected light.